This invention relates generally to hydrostatic drives, and in particular to a new type of electrical control for operating mechanical equipment powered by hydrostatic drive. The control compensates for normal drive and equipment performance variations caused by changes in temperature, lubrication, load, wear, or other miscellaneous factors.
A basic hydrostatic drive, or transmission as it is also known, contains a driving means and all controls required for performing work on a load in one simple package. It provides all the advantages of a conventional hydraulic system, such as stepless adjustment of speed, torque and power, accompanied by smooth and controllable acceleration, the ability to be stalled without damage to the drive, and easy controllability, all within the convenience of a single package procurement and installation.
Early hydrostatic drives were intended primarily for low-cost applications such as farm equipment and garden tractors. Improvements in design, and with control systems in particular, have greatly enlarged the potential applications for such drives in recent years. Accordingly, hydrostatic drives are not used in power-shift transmissions, machine-tool drives, winch drives, concrete mixers and pipe tensioners. They are finding ever-increasing applications in the material handling area with automation of devices such as D-stackers, loaders and unloaders, stampers, elevators, and conveyors. Recent interest in such drives has even been shown for military tank transmissions and possibly for automotive applications.
As previously stated, part of the reason for the increasing attractiveness of hydrostatic drives is improvements in design of hydraulic components as well as control systems. This increased attention to hydrostatic drives is due in part to their great versatility. For example, output performance of these drives can be changed through the provision of either constant or variable output power and torque. Their components may be arrayed in a number of different circuits, including an open circuit used for only one direction rotation, an integral circuit used when all components are contained in a single housing, and a split circuit used when most of the components are separate from the pump housing. In design or selection of a suitable drive for a particular use, applicable considerations include the desired speed regulation of the drive, its variation ratio, its starting torque and accompanying ratio, its service factor, and its jogging and threading capacity.
Improvements in controls have also helped to broaden the field in which hydrostatics are applied. For example, pressure compensators are now available to reduce heat generation, eliminate the necessity for cross-port relief valves and simplify other control circuits. Some load-sensing controls are available with overrides to adjust pump displacements. Brake and bypass circuits to eliminate mechanical braking, power limiters to eliminate prime-mover stalling, and speed controls to eliminate output speed variations have also been developed.
The typical operating cycle for a hydrostatically powered device is characterized as involving discrete portions, or stages, of acceleration followed by travel and then deceleration followed by travel. At that point, the cycle may stop or it may reverse or repeat, depending upon the particular hydrostatic device and application being considered. Of particular significance in this operating cycle, is the travel interval following load deceleration. This interval needs to be controlled for at least three reasons: The interval must be adequate to allow the load to stablize after the shock of deceleration. The interval must be adequate to allow for deviation from the ideal or expected movement of the load which may result from mechanical play, reduced pump or motor efficiency at low volume, system leakage, or the like. The interval must be minimized in order to maintain low equipment cycle times as traveling at low speed consumes significant amounts of potential productive time.
Typical drive controls in use today, whether sensing pressure, speed, brake and bypass, power or load variations, assume that the absolute, or actual, motion of the load follows an ideal and expected movement, as more fully described and depicted in the specification to follow. However, numerous thermal and mechanical changes in temperature, lubrication, load, wear and other miscellaneous factors cause variation from this ideal movement which affects normal drive and equipment performance. Of particular significance are changes affecting the deceleration cycle, since the resulting deviations directly affect movement of the load at the critical time of its final travel interval when it is moving at low speed, and may be about to perform some critical function such as gaining control of, releasing, or positioning some part in a productive process. Notwithstanding this fact, applicant is aware of no prior disclosure, patented or otherwise, that addresses this problem and suggests a means for compensating or correcting for it.